Uranyl Nitrate Hexahydrate
Properties
| State | Solid (hygroscopic) |
| Color | Bright yellow to yellow-green |
| Solubility | Very soluble in water; soluble in nitric acid, alcohols, ethers |
| Melting Point | 60 °C (hexahydrate decomposes) |
About Uranyl Nitrate Hexahydrate
Uranyl nitrate hexahydrate is the bright yellow-green crystalline salt that anchors actinide separation chemistry. The chromophore is the linear trans-dioxo uranyl cation UO2^2+ — a species in which two axial oxygens are formally triple-bonded to a hexavalent U center, leaving four equatorial coordination sites that bind nitrates, water, and most other oxygen donors. The yellow-green color and characteristic vibronic emission around 520 nm under 365 nm UV excitation come from low-lying ligand-to-metal charge-transfer states; this is the same fluorescence that made 19th-century Bohemian "vaseline glass" so eerily luminescent under UV lamps and that still marks U(VI) presence in any analytical spectrum. The hexahydrate is highly soluble in water (about 660 g/L), in nitric acid up to 14 M, and in many oxygenated organic solvents — and one specific organic, tributyl phosphate (TBP) diluted to 30 vol% in dodecane or kerosene, has dominated industrial actinide separations for 70 years. The PUREX process exploits exactly this property: dissolve spent reactor fuel in 8 M HNO3, contact with TBP/kerosene, and UO2(NO3)2 plus Pu(NO3)4 partition into the organic phase as neutral TBP adducts UO2(NO3)2(TBP)2 and Pu(NO3)4(TBP)2 while fission products stay in the aqueous raffinate. Selective reduction of Pu(IV) to Pu(III) then strips plutonium back to the aqueous phase, separating the two actinides cleanly enough for fuel fabrication.
Where you'll encounter it
If you have ever seen a Manhattan Project oral history or visited the historic Hanford B Reactor museum, the bright yellow solutions in the photos of plutonium production chemistry are uranyl nitrate, the dissolved form that gets fed into the bismuth phosphate and later REDOX and PUREX separation flowsheets. At the modern industrial scale, La Hague in France reprocesses about 1,700 tonnes of spent fuel per year using PUREX with uranyl nitrate as the working species in the U-cycle. In a research lab, uranyl nitrate is the go-to reagent for U(VI) coordination chemistry, sol-gel synthesis of UO2 fuel kernels, and uranyl-staining of biological specimens for transmission electron microscopy — a classic technique still used despite the radiological inconvenience because uranyl ion binds tightly to phosphate groups in nucleic acids and to carboxylates in proteins, providing exceptional contrast at sub-nanometer resolution that no organic stain can match. Many older EM facilities have kept their uranyl acetate and uranyl nitrate stocks on a separate licensed shelf for decades.
Common Uses
- Working species in PUREX spent-fuel reprocessing flowsheets at La Hague, Sellafield, and Mayak
- Starting material for sol-gel synthesis of UO2 fuel kernels for TRISO and MOX particle fuel
- Analytical reference standard for uranium quantitation by ICP-MS, alpha spectrometry, and TIMS
- Heavy-metal stain for transmission electron microscopy of biological specimens at sub-nanometer resolution
- Research reagent for uranyl coordination chemistry and actinide-ligand bonding studies
- Historical photographic toning agent producing brown and orange tones in albumen prints
- Pigment for 19th-century vaseline glass and Fiestaware ceramic glazes (now discontinued)
- Calibration material for radiochemistry training and licensed-facility analytical method validation
Safety Information
HAZARDOUS - RADIOACTIVE AND CHEMICALLY TOXIC. GHS: Acute Toxicity Category 2 (oral, inhalation), Carcinogen Category 1B, Reproductive Toxicity Category 1B, Specific Target Organ Toxicity Repeated Exposure Category 2 (kidney), and Aquatic Chronic Hazard Category 1. Like most U(VI) compounds, the dose-limiting hazard at natural and depleted enrichments is chemical kidney toxicity rather than radiological dose. Regulated by NRC under 10 CFR 40 as source material with specific licensing thresholds, and by DOT as Class 7 radioactive plus Class 5.1 oxidizer (the nitrate). Transport classification is UN 2980 for solutions or UN 2912 for solids. Spill response uses absorbent and immediate radiochemistry survey; never mix with reducing agents, organic combustibles, or strong bases without containment. The historical use in vaseline glass, Fiestaware (red and orange colors), and dental porcelain was discontinued in the U.S. by 1943 and replaced postwar with depleted uranium under tighter controls; remaining vintage pieces test positive on a Geiger counter and should not be used for food contact.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.